Moscow, meanwhile, says it will protect the interests of Russian citizens and “compatriots” – wherever they happen to live.
But as the border region teeters on the brink of becoming a battleground, what is Crimea’s status, and why is it so important to both sides?
Politics and population
Crimea is a semi-autonomous region of Ukraine which is largely self governed.
It is normally slated as being pro-Russian due to its demographics – 58 per cent of the region’s 2 million inhabitants are ethnic Russian, while about a quarter are ethnic Ukrainians.
The remaining population are Crimean Tartars, who were deported from the region by former Soviet leader Josef Stalin in 1944 and the subsequent hardships suffered by the Muslim group, many of whom did not survive the exile to Central Asia and Siberia, means that they remain vehemently anti-Russian.
The Tartars also have their own, unofficial, parliament called the Mejlis of the Crimean Tatar People. It was founded in 1991 to act as a representative of the Tartars to the Ukrainian government.
More than half a million people were killed in the Crimean War of 1853-56 between Russia and the Ottoman Empire, which was backed by Britain and France. The conflict reshaped Europe and paved the way for World War I.
In 1921, the peninsula, then populated mainly by Muslim Tatars, became part of the Soviet Union.
Following the deportation of the Tartars at the end of World War II for alleged collaboration with the Nazis, Crimea became part of Russia under the Soviet Union banner.
In 1954, Khrushchev made Crimea part of Ukraine – a move many still see as illegitimate – but then, in 1992 Crimea voted in a referendum to join Ukraine after the fall of the Soviet Union.
In 1996 Crimea was given the status of an autonomous republic on the condition its laws fell in line with those of Ukraine.
Crimea, therefore, occupies a difficult political position, acting as a buffer between its sovereign nation and Russia, a country with which it shares a strong historical association having formally been part of both the Russian Empire and the Soviet Union.
It has been the subject of sporadic tussles between Moscow and Kiev since 1991, but has remained Ukrainian.
On March 16 it voted in a referendum to become part of Russia once again – a vote seen as illegitimate by the US and EU, but seen as being “within international law” by Russia.
The region also holds a modern significance to Russia due to it housing the Russian naval base at Sevastopol – which earlier February 2014 installed a fiercely pro-Russian mayor.
The Black Sea base gives Moscow military access into the Mediterranean and so is of great importance to Russia’s status as a world power. Ukraine’s fleet is also based there.
An agreement was forged between the two countries allowing the Russian fleet to remain at the Black Sea base until 2017 – a deal that was extended by 25 years (until 2042) by Yanukovich in 2010 in return for cheaper gas.
The Ukrainian government placed restrictions within the agreement and Russia has since upgraded its own Black Sea port of Novorossisysk to take naval vessels.
Much of the Black Sea coastline, however, is held by NATO allies such as Turkey, or by countries currently seeking NATO membership – like Georgia.
Geography and economy
Crimea is a peninsula attached to the rest of Ukraine by a narrow strip of land in the north.
To the east, it is separated from Russia by the narrow Kerch Strait, over which Russia plans to build a bridge.
It is Ukraine’s only formally autonomous region, with Simferopol as its capital, covering an area of about 27,000 sq km.
Crimea’s temperate climate makes it a popular tourist destination for Ukrainians and Russians, especially Yalta, where the Soviet, US and British victors of World War Two met in 1945 to discuss the future shape of Europe.